This invention relates to the well known general field of phosphate conversion coating of metals and more particularly to phosphate coatings formed from a liquid phosphating composition that contains both zinc and at least one of nickel, cobalt, and manganese as layer forming cations. The coatings formed from such a phosphating composition normally contain both zinc and at least the one(s) of nickel, cobalt, and zinc also present in the phosphating compositions. These coatings may also contain iron, particularly if a ferriferous substrate such as ordinary (non-stainless) steel is being phosphated.
Almost all phosphating compositions and processes are subject to the formation of “sludge”, a solid phase that separates spontaneously from the liquid phosphating composition as the latter is used. The major components of sludge are water-insoluble phosphates, usually of more or less the same type(s) that constitute the desired conversion coating. Although some attempts have been made to re-use sludge, in most commercial operations it still represents an economically significant cost of phosphating, because the anions and cations incorporated into the sludge generally must be replenished along with the ions from the phosphating composition that actually form the desired phosphate conversion coating. Sludge generally either sinks to the bottom of any container in which it forms or floats on the liquid phosphating composition from which it forms and therefore can be easily removed from the liquid phosphating composition by filtering or skimming if desired or needed. Sludge also is usually only weakly adherent to metal surfaces, and if it does accumulate on them can be readily removed by brushing, water flush, or the like.
A phenomenon less common than sludging that is sometimes observed in commercial phosphating is the formation of an adherent scale on process equipment, such as squeegee rolls, immersion heaters and heat exchangers, that must be kept in contact with the phosphating compositions during their use in order to maintain optimum conditions for phosphating. No phosphate conversion coating of these items of process equipment is desired, and the objects are generally made of non-metals such as rubber for squeegee rolls or of metals such as stainless steel on which normal phosphate conversion coatings do not spontaneously form. Nevertheless, when these objects, especially if their surfaces are hot, are maintained in contact with liquid phosphating compositions for extended periods of time, a relatively hard, adherent, and difficult to remove scale develops over the part of the surface in contact with the phosphating composition. Such scale is usually a heat insulator, so that even a relatively thin coating of the scale substantially impedes the heat transfer, between the metal and the phosphating compositions, that is a major reason for maintaining many of the metal surfaces in contact with the phosphating composition in the first place. On some other surfaces, such as squeegee rolls, the scale can interfere with the intended operation of the process equipment in other ways. The scale must therefore be periodically removed, often as much as every few hours of operation, and scale must usually be removed primarily by hand labor. Its removal therefore is often very costly.
In many commercial phosphating operations, particularly continuous operations such as those usually used to phosphate large metal coils, there is a large fixed capital cost of the equipment used for the phosphating, so that it is economically important to obtain the phosphate coatings rapidly, thereby diminishing the fixed cost per item of production by distributing this cost over more items. In most instances, phosphating reactions proceed more rapidly at higher rather than lower temperatures. A high phosphating temperature is therefore desirable to minimize fixed costs per production item, but if the high temperature causes more rapid scaling as it usually does when the phosphating composition used has a tendency to form scale, the cost of scale removal may destroy the economic benefit of faster phosphating.
Phosphating compositions with a high total concentration of cations of divalent nickel, divalent cobalt, and/or divalent manganese (these three types of cations being hereinafter usually jointly referred to as “NCM”) along with zinc, as taught in U.S. Pat. No. 4,681,641 of Jul. 21, 1987 to Zurilla et al., often provide better corrosion resistance to the metal substrates covered with them than do almost any other kind of commonly used phosphating. However, they are also more prone to sludging and, when the total NCM content is very high, are much more prone to scaling than almost any other type of commonly used phosphating process.
Accordingly, a major object of this invention is to provide high NCM phosphating compositions and/or processes that produce less sludge and/or scaling than previously used high NCM phosphating, particularly when the processes are operated at high temperatures.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, throughout this description, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer”, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ by chemical reactions specified in the description, and does not necessarily preclude other chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole (any counterions thus implicitly specified should preferably be selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to the objects of the invention); the term “paint” and all of its grammatical variations are intended to include any similar more specialized terms, such as “lacquer”, “varnish”, “electrophoretic paint”, “top coat”, “color coat”, “radiation curable coating”, or the like and their grammatical variations; and the term “mole” and its grammatical variations may be applied to elemental, ionic, and any other chemical species defined by number and type of atoms present, as well as to compounds with well defined molecules.